1. Field of the Disclosure
The present disclosure relates to power lines for distribution of electrical power, and more particularly, to an evaluation of noise and excess current on the power lines, for the sake of identifying a location of a source of the noise or excess current.
2. Description of the Related Art
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, the approaches described in this section may not be prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
In a power grid, buried medium voltage (MV) cables and high voltage (HV) cables degrade over time, especially early models that did not have a protective outer insulating cover and were buried directly in the ground. Such cables often have a coaxial cross-section. An exposed outer neutral sheath may be completely eaten away through chemical interaction with minerals in the earth, thus creating an open circuit in the neutral sheath. The open circuit does not necessarily create an outage, as the earth itself may be conductive enough to complete the circuit. However, this is an unintentional, undesirable, fallback.
These cables are often of an Underground Residential Distribution (URD) type. Replacing such a cable includes digging a trench parallel to the cable, laying and terminating a new cable, and creating a brief outage during which the new cable is connected to the power grid.
There is a large number of such cables. The large number of cables, and economic limitations of utility companies, precludes a timely and systematic program of replacing the URD cables in most localities. Instead, in large measure, power utilities respond to outages. Restoring power after an outage requires a process that may take many hours, and often includes becoming aware of an outage from customer complaints, concluding that an outage is due to a distribution fault, dispatching a line crew, locating the fault, and then performing the replacement process described above. This process lengthens outage duration and often employs expensive emergency crews.
Fault location after an outage is aided by electromagnetic over-current trip detectors, where these are installed on URD cables throughout an underground feeder loop. Transformer covers are opened and a detector is observed, until a last tripped detector is located, indicating a fault location. Alternatively, a feeder cable may be disconnected from its power source, and a type of time domain reflectometry termed “thumping” is used to determine a distance of an anomaly from a cable end.
Partial discharge (PD) is a phenomenon that occurs in insulation that has sustained damage, such as through aging, physical damage, or exposure to excessively high electric fields. PD may afflict cables, connectors, surge arrestors, and other high voltage devices. PD generates short pulses, whose duration is in the nano-second range or shorter. PD pulses tend to occur at certain phases of an AC power voltage, and tend to be roughly synchronized with the power frequency or twice the power frequency. PD is a member of a class of noise known as line-synchronized noise or line-triggered noise. PD pulses have a continuous broadband spectrum spanning at least a range between kilohertz and hundreds of megahertz.
PD in cables, connectors, splices, transformers and other hardware may be a precursor of faults. Similarly, arcing may be a precursor, and may also degrade power quality by introducing flicker and damaging surges. Where feeder degradation is suspected, line crews often use handheld PD detectors. Such detectors may be configured with an open core inductive coupler and a specialized radio frequency receiver or ultrasonic detector. The detector is placed onto a cable, and a visual and/or audio indication of PD signals is provided.
While power cables are designed to carry power frequencies, their construction also provides an ability for them to carry radio frequency signals, such as generated by PD, albeit with a rate of attenuation per meter that increases with increasing frequency. This attenuation is a fortuitous characteristic that can help pinpoint a location of PD and arcing, both of whose signals have broadband spectra that includes radio frequencies.
U.S. Pat. No. 7,532,012 describes a multiple frequency PD detector and monitor, and also describes methods for determining whether a radio frequency signal is synchronized with power frequency, thus pointing to PD or arcing, or whether signals are unsynchronized with power frequency and thus unrelated to PD and arcing. A signal derived from a PD pulse is routed through a plurality of channels, each of which has a different bandpass frequency, so that thereafter, a frequency spectrum of the PD pulse can be analyzed.
There is a need for a technique that will alert a person of a defect in a power cable prior to an occurrence of a fault that would lead to an unscheduled power outage, and will help identify the location of the fault if the fault occurs.